1. Field of the Invention
The present invention generally relates to a living tissue stimulator and, more particularly, to an improved hermetically sealed implantable living tissue stimulator.
2. Description of the Prior Art
Presently various stimulators are available commercially for providing stimulating pulses to various living tissue. Among the most widely known is the cardiac pacemaker which is used to provide stimulating pulses to a patient's heart in order to regulate the heart beat. In U.S. Pat. No. 3,867,950 issued on Feb. 25, 1975, a rechargeable cardiac pacemaker which is implantable in a body is disclosed. The advantage of such a pacemaker is that its source of power, typically a battery, can be recharged by an external alternating magnetic field, so that the pacemaker does not have to be removed periodically from the body for battery replacement, thus eliminating the need for frequent surgical operations.
As is appreciated, the body saline fluid is electrically conductive. In addition, it tends to impregnate encapsulating material, e.g., epoxy which is often used to encapsulate the components of an implantable pacemaker. Body fluid, when coming in contact with the pacemaker components tends to corrode and electrically short the components, thereby affecting the operation of the pacemaker. This problem was overcome in the prior art by sealing the pacemaker components, including the pulse generating circuitry, the battery and the recharging circuitry in a hermetic metal container which is impervious to the body fluid. The metal container is typically formed of a biocompatible metal of a thickness on the order of 10 mils (1 mil = 0.001 inch) or more.
Although the prior art hermetically sealed rechargeable pacemaker operates quite satisfactorily, it has significant disadvantages. From experiments therewith it was found that only a very small portion of the power induced in the pacemaker by the external alternating magnetic field is actually converted into useful battery charging power. Most of the induced power is dissipated as heat, particularly in heating up the hermetic metal container. This is undesirable for several reasons. Since the portion of the total induced power converted into useful battery charging power is very low the battery has to be charged more frequently and for longer periods of time than would have been the case if the charging efficiency were greater. Also, since the metal container heats up it may cause patient discomfort and/or damage to body tissue, unless the heating is controlled.
It is realized that the power actually converted to useful battery power may be increased by increasing the intensity of the external alternating magnetic field to increase the total power induced in the stimulator. However, such increase will further increase the heat dissipated by the metal container, which is most undesirable. Thus, a need exists for an improved implantable hermetically sealed pacemaker which is rechargeable by an external alternating magnetic field at a higher efficiency than hereinbefore attained. Also a need exists for an improved implantable hermetically sealed pacemaker in which the container heating due to an external alternating magnetic field is held to a minimum.